The invention relates generally to plasma processing of substrates. In particular, the invention relates to a enhancing the plasma for processing a substrate, most particularly for plasma sputtering a metallic layer on a substrate.
The continuing miniaturization of integrated circuits has been accomplished in large part by the decreasing sizes of the elements of the integrated circuit. At the present time, the minimum lateral feature sizes of integrated circuits for advanced applications is about 0.25 xcexcm and is being pushed rapidly downward to 0.13 xcexcm and even 0.11 xcexcm. At the same time, the thickness of inter-level dielectric layers is being maintained in the region of about 1 xcexcm. As a result of this interaction between shrinking lateral dimension and nearly constant vertical dimension, the aspect ratio of the via or contact holes interconnecting two levels of the integrated circuit is rapidly increasing from 4:1 to 10:1. Filling metal into such a high aspect-ratio hole is a major technological problem.
Sputtering or physical vapor deposition (PVD) is the favored technology for metal hole filling because of its rapid deposition rate and relatively low cost of equipment. However, PVD is not inherently suited to filling of deep, narrow holes because of its generally ballistic nature and nearly isotropic pattern of deposition, which do not foster effective filling of the bottom of such high aspect-ratio holes. Nonetheless, it has been recognized that deep hole filling by PVD can be achieved by ionizing the sputtered atoms and, one way or the other, electrically biasing the wafer being sputter coated so that the ionized sputtered ions are accelerated towards the wafer in a very anisotropic pattern that reaches deeply into the hole.
At least two methods have been recognized for deep hole filling of metals by increasing the ionization fraction of the metallic sputtered atoms. One method uses various techniques to increase the plasma density adjacent to the sputtering target in a diode sputtering reactor and to extend the plasma further away from the target. These methods often involve either small magnetrons which need to be scanned over the target or complexly shaped targets. Furthermore, the high ionization fractions are achieved only at lower chamber pressures which produces electron temperatures typically smaller than that needed to produce a very high ionization fraction of sputtered atoms.
Another method involves inductively coupling RF energy into a plasma source region at least somewhat remote from the wafer being sputter deposited in a reactor otherwise generally configured as a diode reactor. For sputtering, this usually involves an inductive coil wrapped around the periphery of the processing space between the target and the wafer. Such inductive coupling allows a large amount of RF power to be coupled into an extended plasma source region. The combination of a high-density plasma extending over a significant distance and moderate pressures thermalizes the sputtered metal atoms and then ionizes them.
However, inductively coupling RF power into a sputtering reactor for depositing a metal presents some fundamental problems. If the RF induction coil is located outside the chamber, the chamber wall must be dielectric so that it does not short out the RF power. Dielectric chamber walls are available, such as of quartz, but such ceramic walls are generally poorly suited to the extreme vacuum requirements of sputtering, in the neighborhood of 10xe2x88x928 Torr. Chamber walls for sputtering reactors are preferably composed of stainless steel, but stainless steel is moderately conductive electrically and would tend to short out RF field induced across it. Furthermore, sputtering metals such as aluminum and copper invariably coats some of the metal onto the chamber walls so that even a dielectric wall becomes conductive after extended use.
As a result, the preferred conventional configuration places the RF coil within the vacuum chamber adjacent to the source plasma it is generating. For sputtering, the coil is usually wrapped around the cylindrical space between the target and the wafer being sputter coated. But, such a configuration presents inherent problems. The RF coil must penetrate the vacuum chamber. More importantly, the RF coil dissipates a large amount of RF power and absorbs energy from the plasma and must therefore be cooled if its temperature is to be maintained below 1000xc2x0 C. Cooling within a high-vacuum chamber is always difficult particularly when it involves parts that are highly biased electrically.
Accordingly, it is desired to generate a high density plasma without placing an inductive coil inside the chamber.
The invention can be summarized as a plasma reactor, more particularly a DC magnetron sputter reactor having a negatively biased target, in which one or more tubes external to the reactor are connected to pairs of ports disposed across the processing space of the reactor. When a plasma is excited in the reactor, the plasma extends into the tubes. Each tube defines a plasma current loop through the tube and across the processing space of the reactor. An RF power source powers an inductive coil that is magnetically coupled to the current loop within the tube. That is, the inductive coil is a primary coil and the tube is a secondary coil of an electrical transformer.